1. Field of the Invention
The present invention relates to a method for producing an aldehyde useful as e.g. an intermediate for organic syntheses. More particularly, it relates to a method for producing an aldehyde by a hydrogenation reaction of a carboxylic acid or its alkyl ester.
2. Discussion of Background
Heretofore, various methods have been reported for the production of aldehydes. As the most commonly employed method using a carboxylic acid or its derivative as the starting material, so-called Rosenmund reduction method via a carboxylic acid chloride may be mentioned. However, this method has a drawback that the production costs are high.
It is most preferred if it is possible to efficiently produce an aldehyde by reducing a carboxylic acid directly with molecular hydrogen. However, such a method has been believed to be extremely difficult. Namely, with respect to the hydrogenation reaction of a carboxylic acid or its derivative, a method of using yttrium oxide as the catalyst (see U.S. Pat. No. 4,328,373) or a method of using aluminum oxide (see U.S. Pat. No. 3,935,265) has, for example, been reported. However, these methods have a problem with respect to the catalytical activity and selectivity for the aldehyde.
It has been reported that when a cycloalkyl ester of a lower fatty acid such as acetic acid or n-butyric acid is hydrogenated by using complex oxide of zinc and chromium as a catalyst, the corresponding acetaldehyde or n-butyl aldehyde was formed although in poor yield (see Japanese Examined Patent Publication No. 38410/1972), and it has been reported also that when methyl benzoate, pivalic acid or the like is hydrogenated by using iron oxide containing a small amount of chromium oxide, as a catalyst, the corresponding aldehyde will be formed in a yield of certain degree (see European Patent No. 304853). With these catalysts, it is difficult to attain practically satisfactory yield of aldehydes, although the hydrogenation reaction proceeds to some extent.
The present inventors have previously reported on a method for producing an aldehyde by a hydrogenation reaction of the corresponding carboxylic acid or its ester by means of a catalyst composed essentially of zirconium oxide and slight amount of chromium e.t.c. (see U.S. Pat. No. 4,613,700 and Japanese Unexamined Patent Publications No. 152434/1985 and No. 115043/1986).
By this method, when an aliphatic carboxylic acid having two hydrogen atoms bonded to the carbon at the .alpha.-position to the carboxyl group, or its ester, was used as the starting material, a ketone product was produced as a byproduct by decarboxylation condensation, whereby the selectivity for the desired aldehyde was not necessarily adequate. Also in the case where a heterocyclic carboxylic acid containing a hetero atom such as N, S or O in the heterocyclic ring, or its ester, was used as the starting material, the selectivity for the aldehyde was also inadequate. Thus, it has been desired to develop a catalyst with a higher performance suitable for commercial purpose.
As described above, according to the reports available heretofore, a method for efficiently producing aldehydes by directly hydrogenating various carboxylic acids of aliphatic, alicyclic, aromatic or heterocyclic type, or their derivatives, has not yet been established, and there are many problems to be solved, such as improvement of the catalytic activities and the selectivity for the desired products, and prolongation of the effective life of the catalysts.